|Bowser's Star Reactor|
Right at the beginning you'll notice the Bowser music from Super Mario 64. Great, seeing that it's one of the best songs of the classic title. You'll have to go across the linear path ahead. Jump over the gap and dodge the fire. You'll notice a 1-up: get it if you wish, though chances are you won't die much here!
Eventually you'll come across a fire spewing robot and a Thwomp. Wait until the Thwomp is above ground and the robot is finished spewing to go across. Go onto the green platform, which will needlessly make an awesome loop. When it stops, head right. Jump so that Mario touches the pink gravity changing area, and you'll now be walking on the ceiling. Jump over the fire spire and wait until the platform comes your way and jump on it. Next, jump back onto the ceiling, dodge the fire bar and continue right. Dodge the Thwomp, and onto the yellow platform.
Jump over the rotating fire bars and get the 1-Up Mushroom if you feel like it. Go under the next large Thwomp and dodge the oncoming fire. If you need some coins, wait for this next platform to go down and collect them. The next part features a whole bunch of rotating fire bars once again. I've found that it's nearly impossible to get past them, though there's probably a way. Get the coins if needed and Bowser will appear.
He'll start shooting fire balls on the platforms, so quickly get off those platforms as they'll eventually fall down below. Get the Life Shroom in the crystal (quickly) and head on up all the way. A short cut-scene will occur and Bowser will send you up to the planetoid above.
Bowser will attack by jumping then crashing back down to the planet multiple times, causing ripples to go through the planet. If you get caught in one of them, you'll get hurt, so dodge them all. If you're having a hard time doing so, then just jump on one of the green platforms jutting out the ground, as the ripples won't go there. Eventually he'll go far above the planet, and then attempt to land where you are. This is your chance! Try to direct him so that he'll land on one of the blueish glass sections of the planet. If he does, then the glass will break, and he'll be brutally burned by the fire that is under the glass. He'll star to run around the planet. Go in the opposite direction that he's going so that you'll go head on. When he gets near you, he'll turn the other way, though before he gets too far spin attack him. Shortly afterwards spin attack him again.
Next, he'll start to spew more fire balls across the screen and do the other attacks as well. Do the same tactic as before, though this time when you spin attack him, he'll role up in a ball and go across the stage. Again go in the opposing direction and spin attack once more. The third time you have to spin attack him once, then two more times until you've ultimately killed him.